1. Field of the Invention
The present invention relates to a diaphragm device for use in cameras, such as single-lens reflex camera, lens-shutter camera, video camera, electrostatic still camera or the like.
2. Description of the Related Art
A diaphragm device is used to change an area of an aperture of an optical system in order to adjust quantity of incident light passing into the optical system. From an idealistic viewpoint it is most preferable that the aperture of the diaphragm has a circular shape. This is because out-of-focus state of an image formed through the circular aperture is uniform in all directions and because the circular aperture has minimum proportion of its circumferential length to its area, so that the image is most insusceptible to influence of diffraction. Such circular aperture can be formed by an iris diaphragm having increased number of diaphragm blades. However the cost for manufacturing the diaphragm is expensive and it is difficult to produce such diaphragm at low cost.
With the object of obtaining the diaphragm at low cost, several types of diaphragm having only two diaphragm blades have been proposed. FIG. 1 shows an example of the diaphragm of this kind, in which one of the diaphragm blades has an opening of tadpole shape, including a circular portion of maximum diameter and a tail-like portion extending from a part of the circular portion. FIG. 2 shows another example of the diaphragm of this kind, in which each of the diaphragm blades has an opening of partial square shape. FIG. 3 shows a further example of the diaphragm of this kind in which each of the diaphragm blades has an opening of the shape including a semi-circular portion of maximum diameter and a semi-hexagonal portion extending from a part of said semi-circular portion.
These diaphragm devices have defects as explained below.
The diaphragm device as shown in FIG. 1 is a center-position changing diaphragm in which the center of the aperture area of the diaphragm continually changes its position as the diaphragm is operated.
The diaphragm as shown in FIG. 2 always holds its aperture in the form of a square, so that the center of the aperture area does not change its position.
However, when the image is in out-of-focus state it is deformed in only two directions, owing to the existence of only two tangential directions and, furthermore, the out-of-focus image of a point source is deformed into a square shape, which looks strange.
According to the construction as shown in FIG. 3, the aperture takes the form of rugby-ball, at an intermediate state of the aperture closing prcess, which has a large length-breadth ratio, so that there is substantial difference in degree of defocussing depending upon the direction. Furthermore, the shape of the aperture constantly varies according to the opening size of the diaphragm, so that it is impossible to produce a good image.
Particularly in the case where it is desired to produce a soft-focus effect or an intentional out-of-focus effect at peripheral portion of a picture, it has been difficult to attain such effects uniformly at the peripheral portion of the picture, for example in the breadthwise and vertical directions of the picture.
FIGS. 4 and 5 illustrate another example of the conventional diaphragm device including a pair of diaphragm blades which are movable in opposite directions relatively to each other along a common moving line to control the aperture diameter of the diaphragm. In these figures, 1 is a driving motor for driving the diaphragm blades, 2 is a blade driving lever fixedly connected to the output shaft of said driving motor and having blade driving pins mounted thereon to drive the diaphragm blades, 3 is a base plate slidably supporting the diaphragm blades thereon, and 4 and 5 are the diaphragm blades for adjusting the quantity of light passing through an objective. These diaphragm blades are slidably held on said base plate and connected by the pins to said blade driving lever 2. The diaphargm blades 4 and 5 have cut portions 4a and 5a, respectively, which cooperate to form a diaphragm aperture.
These diaphragm blades 4 and 5 are arranged to be movable in parallel toward or away from each other along a common straight line on a plane at right angle to the optical axis of the photographing lens. Thus the motor 1 serves to rotate the blade driving lever 2, which drives the diaphragm blades 4 and 5 trough the blade driving pins to cause parallel movement of said diaphragm blades.
FIG. 5 shows the fully opened state of the diaphragm device, where the larger diameter portions of the cut portions 4a and 5a come into alignment with each other to open the full diameter of the lens.
In operation of the diaphragm, a voltage signal is fed from a voltage source (not shown) to the driving motor 1. The blade driving lever 2 fixedly connected to the output shaft of the motor 1 is rotated thereby, and the diaphragm blades 4 and 5 connected to said blade driving lever 2 through the pins are moved in opposite directions along the common straight line.
Thus the opening area of the aperature formed by the cut portions 4a and 5a of the diaphragm blades 4 and 5 is decreased until the opening area reduces to its minimum diameter.
According to the conventional diaphragm device as described above, the aperture formed by the diaphragm blades 4 and 5 has a square shape having two sides formed by one of the diaphragm blades and other two sides formed by the other diaphragm blade. Although this type of the diaphragm device produces a satisfactory image when it is in in-focus state, it has a defect as explained above with reference to FIG. 2. That is, when the image is in out-of-focus state, the image of a point source is deformed into a square shape, owing to the square luminons flux. A square shape is in line symmetry and, consequently, an out-of-focus image is deformed in two directions only, and thus a very poorly deformed image is produced.